The present invention relates to the extraction of mineral values from mineral-containing materials. In a more specific aspect, the present invention relates to the extraction of mineral values in situ from subsurface formations. In a still more specific aspect, the present invention relates to the extraction of uranium values in situ from subsurface formations containing uranium.
Numerous minerals are present in subsurface earth formations in very small quantities which make their recovery extremely difficult. However, in most instances, these minerals are also extremely valuable, thereby justifying efforts to recover the same. An example of one such mineral is uranium. However, numerous other valuable minerals, such as copper, nickel, molybdenum, rhenium, silver, selenium, vanadium, thorium, gold, rare earth metals, etc., are also present in small quantities in subsurface formations, alone and quite often associated with uranium. Consequently, the recovery of such minerals is fraught with essentially the same problems as the recovery of uranium and, in general, the same techniques for recovering uranium can also be utilized to recover such other mineral values, whether associated with uranium or occurring alone. Therefore, a discussion of the recovery of uranium will be appropriate for all such minerals.
Uranium occurs in a wide variety of subterranean strata such as granites and granitic deposits, pegmatites and pegmatite dikes and veins, and sedimentary strata such as sandstones, unconsolidated sands, limestones, etc. However, very few subterranean deposits have a high concentration of uranium. For example, most uranium-containing deposits contain from about 0.01 to 1 weight percent uranium, expressed as U.sub.3 O.sub.8 as is conventional practice in the art. Few ores contain more than about 1 percent uranium and deposits containing below about 0.1 percent uranium are considered so poor as to be currently uneconomical to recover unless other mineral values, such as vanadium, gold and the like, can be simultaneously recovered.
There are several known techniques for extracting uranium values from uranium-containing materials. One common technique is roasting of the ore, usually in the presence of a combustion supporting gas, such as air or oxygen, and recovering the uranium from the resultant ash. However, the present invention is directed to the extraction of uranium values by the utilization of aqueous leaching solutions. There are two common leaching techniques for recovering uranium values, which depend primarily upon the accessibility and size of the subterranean deposit. To the extent that the deposit containing the uranium is accessible by conventional mining means and is of sufficient size to economically justify conventional mining, the ore is mined, ground to increase the contact area between the uranium values in the ore and the leach solution, usually less than about 14 mesh but in some cases, such as limestones, to nominally less than 325 mesh, and contacted with an aqueous leach solution for a time sufficient to obtain maximum extraction of the uranium values. On the other hand, where the uranium-containing deposit is inaccessible or is too small to justify conventional mining, the aqueous leach solution is injected into the subsurface formation through at least one injection well penetrating the deposit, maintained in contact with the uranium-containing deposit for a time sufficient to extract the uranium values and the leach solution containing the uranium, usually referred to as a "pregnant" solution, is produced through at least one production well penetrating the deposit. The present invention is directed to the latter, "in situ" leaching.
The most common aqueous leach solutions are either aqueous acidic solutions, such as sulfuric acid solutions, or aqueous alkaline solutions, such as sodium carbonate and/or bicarbonate.
Aqueous acidic solutions are normally quite effective in the extraction of uranium values. However, aqueous acidic solutions generally cannot be utilized to extract uranium values from ore or in situ from deposits containing high concentrations of acid-consuming gangue, such as limestone. Aqueous alkaline leach solutions are applicable to all types of uranium-containing materials and are less expensive than acids.
The uranium values are conventionally recovered from acidic leach solutions by techniques well known in the mining art, such as direct precipitation, selective ion exchange, liquid extraction, etc. Similarly, pregnant alkaline leach solutions may be treated to recover the uranium values by contact with ion exchange resins, precipitation, as by adding sodium hydroxide to increase the pH of the solution to about 12, etc.
As described to this point, the extraction of uranium values is dependent to some extent upon the economics of mining versus in situ extraction and the relative costs of acidic leach solutions versus alkaline leach solutions. However, this is an oversimplification, to the extent that only uranium in its hexavalent state can be extracted in either acidic or alkaline leach solutions. While some uranium in its hexavalent state is present in ores and subterranean deposits, the vast majority of the uranium is present in its valence states lower than the hexavalent state. For example, uranium minerals are generally present in the form of uraninite, a natural oxide of uranium in a variety of forms such as UO.sub.2, UO.sub.3, UO.U.sub.2 O.sub.3 and mixed U.sub.3 O.sub.8 (UO.sub.2.2UO.sub.3), the most prevalent variety of which is pitch blende containing about 55 to 75 percent of uranium as UO.sub.2 and up to about 30 percent uranium as UO.sub.3. Other forms in which uranium minerals are found include coffinite, carnotite, a hydrated vanadate of uranium and potassium having the formula K.sub. 2 (UO.sub.2).sub.2 (VO.sub.4).sub.2.3H.sub.2 O, and uranites which are mineral phosphates of uranium with copper or calcium, for example, uranite lime having the general formula CaO.2UO.sub.3.P.sub.2 O.sub.5.8H.sub.2 O. Consequently, in order to extract uranium values from subsurface deposits with aqueous acidic or aqueous alkaline leach solutions, it is necessary to oxidize the lower valence states of uranium to the soluble, hexavalent state.
Combinations of acids and oxidants which have been suggested by the prior art include nitric acid, hydrochloric acid or sulfuric acid, particularly sulfuric acid, in combination with air, oxygen, sodium chlorate, potassium permanganate, hydrogen peroxide and magnesium dioxide, as oxidants. Alkaline leachants and oxidants or lixivants heretofore suggested include carbonates and/or bicarbonates of ammonium, sodium or potassium in combination with air, oxygen or hydrogen peroxide, as lixivants. However, sodium bicarbonate and/or carbonate have been used almost exclusively in actual practice.
While the previous discussion would indicate that "in situ" recovery of mineral values, such as uranium, is fairly simple and straight forward and would appear to be the best technique in most cases the very nature of subsurface formations containing mineral values and the types of formations in which such mineral values are found seriously complicate "in situ" recovery.
Quite often, subsurface formations containing mineral values are heterogeneous to the extent that the porosity varies considerably in a vertical direction thus having horizontally disposed zones of both high and low porosity, either in direct contact with one another or separated by layers of nonporous or impermeable formations. For obvious reasons, injection and production wells for in situ recovery of mineral values from such formations are completed so as to be in communication with the entire vertical dimension of the formation, rather than individual zones. Accordingly, the injected leach solution, which travels in a generally horizontal direction from the injection well or wells to the production well or wells will follow the path of least resistance, thus preferentially flowing through zones of high permeability with very little flowing through zones of low permeability. Accordingly, recovery of mineral values from zones of low permeability is very limited and in order to increase recovery from such zones of low permeability it is necessary to utilize excessive amounts of leach solution and substantially increase the time necessary for maximizing the recovery of mineral values.